Water-resistance is considered to be an important factor in the production of building products containing gypsum, for example plasterboard, gypsum blocks, and mouldings. Adsorption of water can cause staining, mildew, expansion, and general deterioration and degradation of gypsum products. It is particularly difficult to provide gypsum products which may be utilised on external walls, due to the effects of the weather. Various techniques have been proposed to render gypsum products more hydrophobic, for example by way of surface treatment with a hydrophobing agent, i.e. spraying, brushing or rolling a hydrophobing agent onto a gypsum based substrate, by immersing the substrate in a solution of hydrophobing agent, or by admixing a hydrophobing agent into gypsum powder prior to formation of a gypsum based product, for example prior to moulding blocks or forming boards.
Surface treatment using a hydrophobing agent is a convenient means of increasing the water repellency of a pre-formed substrate, such as a pre-moulded gypsum block or plasterboard. However, after such a treatment the hydrophobing agent tends to be only dispersed in the surface layers of the substrate with the bulk of the substrate material remaining substantially untreated.
EP0592206 describes a method of preparing a water repellent gypsum board by applying a composition comprising water, a surfactant, an Si—H containing siloxane and a high molecular weight silicone.
An alternative method for rendering a substrate water repellent is to admix a hydrophobing agent into substrate material, such as gypsum powder, prior to moulding blocks or forming boards or the like. GB 1581396 describes a water resistant gypsum composition, which was thought to be useful for external applications. The composition was prepared by mixing a methyl hydrogen organopolysiloxane with powdered gypsum and optionally a setting retarder and a thickening agent. The methyl hydrogen organopolysiloxane was mixed into the gypsum in the absence of water and subsequently the required amount of water was added to the resulting mixture. GB 2062607 describes the manufacture of a gypsum-hardened body from a mixed material comprising gypsum, slag and a powdered additive coated with hydrophobic diorganopolysiloxane. U.S. Pat. No. 4,643,771 describes the production of water repellent plaster moulding by forming a foam of water, surfactant and polyalkylhydrogensiloxane, adding the foam to a water paste of plaster powder and allowing the mixture to set. U.S. Pat. No. 4,851,047 discloses a process for preparing water repellent articles from gypsum powder containing hydrophobic agents in which the hydrophobic agents are prepared by spray drying a mixture containing water, organopolysiloxane and a water soluble, film forming, polymer. U.S. Pat. No. 5,336,715 discloses water dilutable organopolysiloxane compositions for use as hydrophobing agents for materials such as concrete, gypsum and paint, which comprises a salt of a water-soluble organic or inorganic acid, an organopolysiloxane having at least one SiC-bonded organic radical containing basic nitrogen, and an alkoxylated alcohol. U.S. Pat. No. 4,704,416, describes aqueous redispersible powders obtained by spray drying an aqueous mixture of a water soluble polymer and at least one organic silicon polymer said powders are said to be utilisable for rendering bulk materials hydrophobic when diluted with water.
It has been considered for a long time by the building trade that while products containing a high proportion of gypsum are suitable for internal building their use in external masonry and rendering products is unacceptable due to the water absorbent properties of gypsum. One reason for this is that whilst the pre-mixing of a hydrophobing additive with gypsum, in the absence of water, results in a substantially uniform dispersion of hydrophobing additive throughout the mixture, upon the introduction of water the hydrophobing agent tends to migrate away from the water towards the gypsum air interface resulting in a concentration of hydrophobic agent on the outer surface of a gypsum block, i.e. at the interface with air. This migration effect results in the presence of very little hydrophobing agent in the inner body of blocks of gypsum and as such it has previously proved very difficult, if not impossible to maintain a uniform dispersion of hydrophobing additive throughout a gypsum block, subsequent to wetting. There still remains a long felt need to be able to make gypsum sufficiently hydrophobic in order for it be used in external wall applications.